The present invention generally relates to an apparatus and methodology for allowing a single battery cell or module to be charged or discharged in parallel within a series circuit, so that the cells or modules can be balanced either during charging, discharge, or at rest (i.e. not being charged or discharged via an outside source.).
There is a growing need for cell balancing because there are inconsistencies in the construction and materials of every cell that while small, can have a material impact on the cell's charge and discharge characteristics. This non-uniformity tends to magnify differences in a cell's State of Charge (SOC) over time when compared to other cells similarly charged and discharged. While balancing cell SOC in a battery can extend the life of the battery, it can also reduce the chance that a cell may experience a thermal event that can range from gassing to combustion. While most battery cells (and hence the battery) display some sort of degradation in performance if over- or undercharged, the Li-ion battery has a history of causing fires if overcharged and their cost makes damaging them in a low SOC an event that is to be avoided.
Traditional battery and battery cell management systems remove a cell from a battery or remove a battery from a pack if and when its SOC crosses an upper or lower threshold. The problem with this strategy is that on occasion, the removal of a cell during discharge causes the other cells to work harder to discharge the required energy, increasing the chances that another cell will reach its limit, which then increases the chance that another cell will reach its limit and so on. Cascade failure occurs when the battery or battery pack sequentially fails and is removed from the system to the point that the whole system shuts down.
More advanced cell management systems sequentially read individual cell voltage at different points of time to determine its SOC. However, as the load (or charge) on a battery increases, the SOC can change materially and non-linearly in very small amounts of time. Individual readings even over short periods of time can lead to inaccurate measurements and thus less efficient balancing. The present system can be set to monitor and review all cells simultaneously and compute corresponding SOC simultaneously. By simultaneously monitoring cell SOC, more precise measurement can be made, and more precise cell management can be attained.
An additional feature of the present system is the ability to group a number of cells in series (a “module”) and to treat the module as an individual cell. This ability can speed the balancing process.
The present system can passively monitor cells and has the ability to latch on to a cell or module when energy movement is required. This can extend battery cell life by powering itself to cycle a cell into the balancing only as required. Other cell balancing systems sequentially latch to each cell in and out of the system to monitor their voltage. As such, the present system can be more energy efficient than existing systems.
For battery manufacturers, vehicle manufacturers, electric grid system manufacturers, the military, and wherever battery electrical power is used, the system disclosed herein can more efficiently manage electrical energy between battery cells, or modules, so that the batteries last longer, remain safer, and are capable of higher performance.
It is expected that as new battery, energy storage, and energy generation technologies evolve; various combinations of existing and future technologies can be balanced using the present system.
A cell management system is disclosed for balancing energy across a plurality of cells coupled to a circuit bus where each cell of the plurality of cells includes a positive terminal and a negative terminal. The cell management system includes a transformer, first and second transformer switches, and for each individual cell: a first cell switch pair and a second cell switch pair. The transformer couples the circuit bus and the plurality of cells. The transformer includes a first inductor coupled to the circuit bus and a second inductor coupled to the plurality of cells. The first transformer switch couples the first inductor and the circuit bus. The second transformer switch couples the second inductor and the plurality of cells. The first cell switch pair of each individual cell allows the transfer of energy between the transformer and the individual cell. The second cell switch pair of each individual cell allows the removal or inclusion of the individual cell in the serial connection of the plurality of cells. The cell management system also includes a connection between the circuit bus and a first cell located at one end of the serial connection of cells, and a connection between the circuit bus and a last cell located at the opposite end of the serial connection of cells.
The cell management system can also include a first diode connected in parallel with the first transformer switch and a second diode connected in parallel with the second transformer switch.
The first cell switch pair for a cell can include a first cell switch coupling the negative terminal of the cell with the second inductor; and a second cell switch coupling the positive terminal of the cell with the second transformer switch. In one embodiment, the second inductor is located between the first cell switch and the second transformer switch, and the second transformer switch is located between the second inductor and the second cell switch.
The second cell switch pair for a cell can include a third cell switch and a fourth cell switch. In one embodiment, if the cell is the first cell, the third cell switch couples the negative terminal of the cell with the circuit bus; otherwise the third cell switch couples the negative terminal of the cell with the positive terminal of the preceding cell in the serial connection of cells. In this embodiment, if the cell is the last cell, the fourth cell switch couples the negative terminal of the cell with the circuit bus; otherwise the fourth cell switch couples the negative terminal of the cell with the negative terminal of the next cell in the serial connection of cells. In this embodiment, the third cell switch couples the fourth cell switch and the negative terminal of the cell, and the third cell switch couples the first cell switch and the negative terminal of the cell.
The second cell switch pair for a cell can include a third cell switch and a fourth cell switch wherein, when the cell is not the last cell, the third cell switch couples the positive terminal of the cell with the negative terminal of the next cell in the serial connection of cells; and when the cell is the last cell, the third cell switch couples the positive terminal of the cell with the circuit bus. In this embodiment, when the cell is not the last cell, the fourth cell switch couples the negative terminal of the cell with the negative terminal of the next cell in the serial connection of cells; and when the cell is the last cell, the fourth cell switch couples the negative terminal of the cell with the circuit bus. In this embodiment, the third cell switch is located between the fourth cell switch and the positive terminal of the cell, and the third cell switch is located between the second cell switch and the positive terminal of the cell.
The cell management system can also include cell sensors, bus sensors, cell state estimators, system state estimators and a controller. The cell sensors can monitor parameter of the cells, for example cell voltage, cell temperature, or cell current. The bus sensor can monitor a parameter of the circuit bus. The cell state estimator can determine a state of each cell using the cell sensors. The system state estimator can determine a state of the circuit bus. The controller can control the first transformer switch, the second transformer switch, the first cell switch pair and the second cell switch pair based on the state of each of the cells and the state of the circuit bus.
The cell management system can include a short-term energy storage device, a storage device switch pair allowing the transfer of energy between the transformer and the short-term energy storage device, and a third cell switch pair for each cell. The third cell switch pair allows the transfer of energy between the short-term energy storage device and the cell. The third cell switch pair can include a fifth cell switch coupling the negative terminal of the cell with the short-term energy storage device, and a sixth cell switch coupling the positive terminal of the cell with the short-term energy storage device, where the short-term energy storage device is located between the fifth cell switch and the sixth cell switch. The storage device switch pair can include a first storage device switch coupling the negative terminal of the storage device with the second inductor and a second storage device switch coupling the positive terminal of the storage device with the second transformer switch, where the second inductor is located between the first storage device switch and the second transformer switch, and the second transformer switch is located between the second inductor and the second storage device switch. The cell management system can also include a storage device state estimator determining a state of the short-term energy storage device, and the controller can control the storage device switch pair based on the state of the cells, the circuit bus, and the short-term energy storage device.
An embodiment of a cell management system for balancing energy across a plurality of cells coupled to a circuit bus is disclosed, where each of the cells includes a positive terminal and a negative terminal. The cell management system includes a first cell switch pair, a second cell switch pair, and a fourth cell switch pair for each cell, a transformer and two transformer switches. The transformer can include a first inductor on a first side of the transformer, and a second inductor on a second side of the transformer. The first cell switch pair for each cell is on the second side of the transformer, and the fourth cell switch pair for each cell is on the first side of the transformer. The first transformer switch is located between the first inductor and the fourth cell switch pair for each cell. The second transformer switch is located between the second inductor and the first cell switch pair for each cell. This embodiment of the cell management system also includes a connection between the circuit bus and a first cell located at one end of the serial connection of cells; and a connection between the circuit bus and a last cell located at the opposite end of the serial connection of cells. The first cell switch pair of a cell allows the transfer of energy between the transformer and the cell, the second cell switch pair of a cell allows the removal or inclusion of the cell in the serial connection of cells, and the fourth cell switch pair of the cell allows the transfer of energy between the transformer and the cell.
The first cell switch pair for a cell can include a first cell switch coupling the negative terminal of the cell with the second inductor, and a second cell switch coupling the positive terminal of the cell with the second transformer switch; the second inductor being between the first cell switch and the second transformer switch, and the second transformer switch being between the second inductor and the second cell switch. The fourth cell switch pair for a cell can include an eighth cell switch coupling the negative terminal of the cell with the first inductor, and a seventh cell switch coupling the positive terminal of the cell with the first transformer switch; the first inductor being between the eighth cell switch and the first transformer switch, and the first transformer switch being between the first inductor and the seventh cell switch.
This embodiment of the cell management system can also include a short-term energy storage device, a storage device switch pair allowing the transfer of energy between the transformer and the short-term energy storage device, and a third cell switch pair for each cell that allows the transfer of energy between the short-term energy storage device and the cell. The cell management system can also include cell sensors, bus sensors, storage device sensors, cell state estimators, a system state estimator, a storage device state estimator and a controller to control the transformer switches, the cell switches, and the storage device switches based on the appropriate sensor readings.
A method is disclosed for controlling a plurality of cells connected to a transformer through a first set of switches to charge and discharge the transformer and connected in series through a second set of switches to provide energy to a system. The method includes determining a state of charge value for each cell; determining a minimum state of charge value for the plurality of cells; determining a minimum state of charge cell having the minimum state of charge value; determining a maximum state of charge value for the plurality of cells; determining a maximum state of charge cell having the maximum state of charge value; calculating a state of charge difference as the difference between the maximum and minimum state of charge values; when the state of charge difference exceeds a state of charge deadband, equalizing the state of charge values of the plurality of cells using the transformer; identifying whether any cells are unhealthy; and bypassing any unhealthy cells.
The step of equalizing the state of charge values of the plurality of cells using the transformer can include determining whether the cells are in a charge mode or a discharge mode. When the cells are in the charge mode, switching at least two switches of the first set of switches to isolate the minimum state of charge cell and to charge the minimum state of charge cell using the transformer. When the cells are in the discharge mode, switching at least two switches of the first set of switches to isolate the maximum state of charge cell and to discharge the maximum state of charge cell using the transformer.
The step of bypassing any unhealthy cell can include determining whether to replace the energy of the unhealthy cell. When it is determined not to replace the energy of the unhealthy cell, switching at least one switch of the second set of switches to remove the unhealthy cell from the serial connection of cells and maintain the serial connection of the remaining cells. When it is determined to replace the energy of the unhealthy cell, switching at least one switch of the second set of switches to remove the unhealthy cell from the serial connection of cells and switching at least one switch of the first set of switches to insert energy from the transformer in place of the unhealthy cell.
A method is disclosed for controlling a plurality of cells connected to a transformer through a first set of switches to charge and discharge the transformer and connected in series through a second set of switches to provide energy to a system. The method includes determining a state of charge value for each healthy cell; determining a minimum state of charge value across all of the healthy cells; determining a maximum state of charge value across all of the healthy cells; computing an average state of charge value for all of the healthy cells; computing a delta minimum state of charge value equal to the difference between the average state of charge value and the minimum state of charge value; computing a delta maximum state of charge value equal to the difference between the maximum state of charge value and the average state of charge value; identifying any overcharged cells; identifying any undercharged cells; determining whether to equalize the state of charge values of the healthy cells; identifying whether any cells are unhealthy; and when an unhealthy cell is identified, bypassing the unhealthy cell.
The step of bypassing the unhealthy cell can include determining whether to replace the energy of the unhealthy cell. When it is determined not to replace the energy of the unhealthy cell, switching at least one switch of the second set of switches to remove the unhealthy cell from the serial connection of cells and maintain the serial connection of the remaining cells. When it is determined to replace the energy of the unhealthy cell, switching at least one switch of the second set of switches to remove the unhealthy cell from the serial connection of cells and switching at least one switch of the first set of switches to insert energy from the transformer in place of the unhealthy cell.
When it is determined to equalize the state of charge values of the healthy cells, the method can include determining whether the delta maximum state of charge value is greater than the delta minimum state of charge value. When the delta maximum state of charge value is greater than the delta minimum state of charge value and any overcharged cells are identified, switching at least two switches of the first set of switches to isolate each of the overcharged cells and to discharge the overcharged cells using the transformer. When the delta maximum state of charge value is not greater than the delta minimum state of charge value and any undercharged cells are identified, switching at least two switches of the first set of switches to isolate each of the undercharged cells and to charge the undercharged cells using the transformer.
When it is determined to equalize the state of charge values of the healthy cells, the method can include the following steps. When both overcharged and undercharged cells are identified, switching at least two switches of the first set of switches to isolate each of the overcharged cells and discharge the overcharged cells using the transformer, and to isolate each of the undercharged cells and charge the undercharged cells using the transformer. When both overcharged and undercharged cells are not identified, determining whether the delta maximum state of charge value is greater than the delta minimum state of charge value; when the delta maximum state of charge value is greater than the delta minimum state of charge value and any overcharged cells are identified, switching at least two switches of the first set of switches to isolate each of the overcharged cells and to discharge the overcharged cells using the transformer; and when the delta maximum state of charge value is not greater than the delta minimum state of charge value and any undercharged cells are identified, switching at least two switches of the first set of switches to isolate each of the undercharged cells and to charge the undercharged cells using the transformer.